ACHANDRASTUDY OF THE ROSETTE STAR-FORMING COMPLEX. III. THE NGC 2237 CLUSTER AND THE REGION'S STAR FORMATION HISTORY

Based on a multiwavelength study, the interstellar medium around the H II region Sh2-173 has been analysed. The ionized region is clearly detected in the optical and the radio continuum images. The analysis of the H I data shows a region of low emissivity that has an excellent morphological correlation with the radio continuum emission. The H II region is partially bordered by a photodissociation region, which, in turn, is encircled by a molecular structure. The H I and CO structures related to Sh2-173 are observed in the velocity ranges from −25 to −31 km s−1 and from −27 to −39 km s−1, respectively. Taking into account the presence of non-circular motions in the Perseus spiral arm, together with previous distance estimates for
\nthe region, we adopt a distance of 2.5 ± 0.5 kpc for Sh2-173. Seven hot stars were identified in the field of Sh2-173, being only one O-type star. The amount of energetic photons emitted by this star is enough to keep the region ionized and heat the dust. Given that an expanding H II region may trigger star formation, a search for young stellar object (YSO) candidates was made using different infrared point source catalogues. A population of 46 YSO candidates was identified, projected on to the molecular clouds. On the other hand, Sh2-173 is located in a dense edge of a large (∼5°) H I shell, GSH 117.8+1.5-35. The possibility of Sh2-173 being part of a hierarchical system of three generations is suggested. In this scenario, the large H I shell, which probably originated due to the action of Cas OB5, would have triggered the formation of Sh2-173, which, in turn, is triggering new stars in its surrounding molecular cloud. To test this hypothesis, the ages
\nof both the H II region and the large shell were estimated and compared. We concluded that Sh2-173 is a young H II region of about 0.6–1.0 Myr old. As for the large shell, we obtained a dynamical age of 5 ± 1 Myr. These age estimates, together with the relative location of the different structures, support the hypothesis that Sh2-173 is part of a hierarchical system.

Rich young stellar clusters produce HII regions whose expansion into the nearby molecular cloud is thought to trigger the formation of new stars. However, the importance of this mode of star formation is uncertain. This investigation seeks to quantify triggered star formation (TSF) in IC 1396A (a.k.a., the Elephant Trunk Nebula), a bright rimmed cloud (BRC) on the periphery of the nearby giant HII region IC 1396 produced by the Trumpler 37 cluster. X-ray selection of young stars from Chandra X-ray Observatory data is combined with existing optical and infrared surveys to give a more complete census of the TSF population. Over 250 young stars in and around IC 1396A are identified; this doubles the previously known population. A spatio-temporal gradient of stars from the IC 1396A cloud toward the primary ionizing star HD 206267 is found. We argue that the TSF mechanism in IC 1396A is the radiation-driven implosion process persisting over several million years. Analysis of the X-ray luminosity and initial mass functions indicates that >140 stars down to 0.1 Msun were formed by TSF. Considering other BRCs in the IC 1396 HII region, we estimate the TSF contribution for the entire HII region exceeds 14-25% today, and may be higher over the lifetime of the HII region. Such triggering on the periphery of HII regions may be a significant mode of star formation in the Galaxy.

Massive stars form on different scales, ranging from large, dispersed OB associations to compact, dense starburst clusters. The question whether one single star formation mechanism is responsible for this wide range of properties is not answered yet. The complex structure of regions of massive star formation, and the involved short timescales provide a challenge for our understanding of their birth and early evolution. In this thesis, I study the formation process of the massive stellar content in two of the most massive and luminous star forming regions in our Galaxy: W49 and W51. I analyse near-infrared (NIR) observations obtained with state-of-the-art ground based telescopes. NIR spectroscopic observations provide reliable classification on the nature of massive stars still heavily embedded in Giant Molecular Clouds (GMCs). With the derived physical properties of the massive stars, we aim to investigate and determine the star formation history of the star forming regions. A very massive star (VMS) (M > 100 M⊙) is discovered in the central cluster of W49. It is classified as an O2-3.5If* star based on its K-band spectrum. By comparing with Geneva stellar evolutionary models, the initial mass of this star is estimated as between 100 M⊙ and 180 M⊙. With the complete spectroscopic observations of W49, thirteen O type stars as well as two Young Stellar Objects (YSOs) are identified. The age of the cluster is estimated as ~1.5 Myr, with star formation still ongoing in different parts of the region. The stellar content of W51 is also studied in this thesis. Evidence has been found that the star formation in W51 has started ~5 Myr ago and still active until now. The distinct environments and properties of sub-clusters are discussed. Despite the fact that the W51 and W49 GMCs have similar mass, we find very different massive stellar population and star formation history. W51 does not contain any VMS, while W49 has 3-4 stars more massive than 100 M⊙. This might be related to differences in the star formation process between these two regions.

view Abstract Citations (34) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The NGC 281 west cluster. I. Star formation in photoevaporating clumps. Megeath, S. T. ; Wilson, T. L. Abstract The NGC281 West molecular cloud is an excellent test case for studying star formation in the clumpy interface between a \hii region and a giant molecular cloud. We present here a study based on new high resolution radio and near-infrared data. Using the IRAM 30-meter telescope, we have mapped the interface in the \cotwo, \coone, and \cs transitions with FWHP beamwidths <= 22''. We have imaged the same region with the VLA in the 20, 6 and 2 cm continuum bands to obtain complementary maps of the ionized gas distribution with angular resolutions <= 13''. In addition, we have obtained near-infrared J and K'-band images to detect young stars in the interface. The 30-meter data shows the molecular gas is concentrated into three clumps with masses of 570, > 210, and 300 \msun and average volume densities of 1.4, >1, and 2 x 10(4) \cm. We detect \cs emission in two of the clumps, indicating peak densities in excess of 5x 10(5) \cm are attained in the clumps. A comparison of the \co line data with the 20 cm continuum image suggests that the molecular clumps are being photoevaporated through their direct exposure to the UV radiation from neighboring OB stars. The luminosity and extent of the observed 20 cm emission is in good agreement with theoretical predictions. We use models of photoevaporative flows to estimate the pressure exerted on the clumps by the ionized gas and find that it exceeds the internal, turbulent pressure of the clumps by a factor of a 2.5. Although a pressure equilibrium is not excluded given the uncertainties inherent in determining the pressures of the ionized and molecular gases, our best estimates of the clumps and flow parameters favor the the existence of low velocity shocks (1.5 \kms) in the clumps. The clumps exhibit broad, non-gaussian lineshapes and complex kinematical structures suggestive of shocks. Further evidence for shocks is found in a comparison of position-velocity diagrams with published numerical simulations of imploding spherical clumps. We discuss the possibility that the knots of \cs emission may trace gas compressed by converging shock waves. The K'-band observations show a rich cluster of primarily low mass stars in the \hii/molecular interface, which we argue is divided into two distinct sub-clusters. We associate one sub-cluster with the two clumps nearest the OB stars, and the second sub-cluster with the third clump. The two clumps nearest the OB stars contain an embedded population, suggesting that star formation is ongoing. We discuss the impact photoevaporation is having on star formation in these two clumps. We find that photoevaporation is dispersing the molecular gas from which the cluster is forming and estimate that the molecular gas will be completely evaporated in 5 Myr. Deep K'-band imaging of the two clumps show that the stars are detected primarily on the sides of the clumps facing the OB stars and in the adjoining \hii region. We examine three explanations for this asymmetry: displacement of the clump centers from the cluster center by the acceleration of the molecular gas through photoevaporation (i.e. the rocket effect), unveiling of young, embedded stars by ionization-shock fronts, and the triggered formation of stars by shocks advancing into the clumps. If shock compression is indeed ongoing in the clumps, then we argue that there is a good case for shock triggered star formation. Publication: The Astronomical Journal Pub Date: September 1997 DOI: 10.1086/118541 Bibcode: 1997AJ....114.1106M Keywords: ISM: INDIVIDUAL: NGC 281; ISM: CLOUDS; ISM: STRUCTURE; STARS: FORMATION full text sources ADS | data products SIMBAD (2)

This thesis comprises the dynamical study of star clusters in the Milky Way and Magellanic Clouds from early to ancient times. Much is unknown about the formation of high-mass stars and clusters — our understanding is deeply hindered by the obscuration of stars by thick columns of dust and gas. One can infer the motions of stars in star-forming regions, however, via radio observations of ionised gas. By way of example, I examine a young, bipolar H II region in the Galactic disc which lies at the centre of a massive (∼ 10^3 M⊙) infrared-dark cloud filament. Intriguingly, the region known as G316.81– 0.06, displays a large velocity gradient (47.81 ± 3.21 km s−1 pc−1) along the same direction as the filament — a phenomenon scarcely observed at this stage of evolution. Based on a qualitative comparison between G316.81–0.06 and simulations of young star-forming regions, the velocity gradient can be explained by rotation, inferred to be a direct result of the initial angular momentum of the natal molecular cloud. If true, this kinematic signature should be common in other young (bipolar) H II regions and may help to discern the scenario by which star clusters form and evolve. Star clusters at ancient times (i.e. globulars) appear in an entirely different form. Rid of their natal gaseous cocoons, globulars visibly contain 10^5 − 10^6 stars, held together by their mutual gravity. One particular conundrum appeared in recent decades: observed mass-to-light ratios (M/L) of metal-rich globular clusters (GCs) disagree with theoretical predictions. This discrepancy is of fundamental importance since stellar population models provide the stellar masses that underpin most of extragalactic astronomy, near and far. Using integral-field unit data from the WAGGS project, I have extracted radial velocities for 1,622 stars located in the centres of 59 Milky Way GCs — twelve of which have no previous kinematic information — in order to calculate dynamical masses and M/L_V ratios via N-body modelling. Most importantly, the sample includes NGC 6528 and NGC 6553, which extend the metallicity range of GCs with measured M/L up to [Fe/H] ∼ −0.1 dex. The results confirm that metal-rich clusters have M/L_V more than two times lower than what is predicted by simple stellar population models, and thus the discrepant M/L– [Fe/H] relation remains a serious concern. I have explored the potential origin of the divergence, and it appears that dynamical effects are the most likely explanation. With great technological advances in recent years, the internal kinematics of more distant star clusters can also be probed, such as massive star clusters in the Magellanic Clouds. These clusters are as young as ∼ 1 Myr and are thought to be the progenitors of ancient globulars. Thus, this provides a unique opportunity for the study of globular formation at a relatively unexplored snapshot in time. I have carried out a preliminary study of eleven (young, intermediate-age and old) massive clusters in the Clouds as an extension of the M/L–[Fe/H] study of GCs. With this, I can then test stellar population models and improve constraints on theories of dynamical evolution at early times. Newly discovered Gaia star clusters present another avenue for novel research. Home to a new area of parameter space, these clusters appear to be old and compact, yet they are faint (V-band magnitude < −2.5 mag). This is an exciting opportunity to advance our knowledge of (heavily dissolved) star clusters which seem to be approaching the end of their lifetime.

Among the most interesting open problems in the theory of star formation is the question whether regions of high-mass star formation also form large numbers of low-mass stars. To answer this question reliably, it is necessary to resolve the crowded central clusters of some of these regions. NGC 3603 is the most massive optically visible giant HII region in our Galaxy. It is located at a distance of 7.2 kpc in the Carina spiral arm. We have observed its central cluster with the SHARP II camera attached to the ADONIS adaptive optics system at the 3.6m telescope on La Silla, Chile. From the photometry of more than 1200 stars, we have constructed near-infrared color-color and color-magnitude diagrams. We find a well-defined main sequence above 4 solar masses; the stars with lower masses have not yet reached the main sequence. By comparison with pre-main-sequence evolutionary models we estimate the age for these lower mass stars to be less than 1 million years. This estimate is supported by the shape of the luminosity function as well. Interpreting the luminosity function in terms of stellar masses, we can construct the initial mass function in NGC 3603 down to about 1 solar mass: We do not find a lower mass cutoff down to this value, which shows that there is indeed prodigious formation of low-mass stars in NGC 3603. The initial mass function — referring to logarithmic mass intervals — follows a power law of index ? = 0.74... 1.32.

We carried out new CO (J = 1–0, 2–1, and 3–2) observations with NANTEN2 and ASTE in the region of the twin Galactic mini-starbursts NGC 6334 and NGC 6357. We detected two velocity molecular components of 12 km s−1 velocity separation, which is continuous over 3° along the plane. In NGC 6334 the two components show similar two-peaked intensity distributions toward the young H ii regions and are linked by a bridge feature. In NGC 6357 we found spatially complementary distribution between the two velocity components as well as a bridge feature in velocity. Based on these results we hypothesize that the two clouds in the two regions collided with each other in the past few Myr and triggered the formation of the starbursts over ∼ 100 pc. We suggest that the formation of the starbursts happened toward the collisional region of extent ∼ 10 pc with initial high molecular column densities. For NGC 6334 we present a scenario which includes spatial variation of the colliding epoch due to non-uniform cloud separation. The scenario possibly explains the apparent age differences among the young O stars in NGC 6334, which range from 104 yr to 106 yr; the latest collision happened within 105 yr toward the youngest stars in NGC 6334 I(N) and I which exhibit molecular outflows without H ii regions. For NGC 6357 the O stars were formed a few Myr ago, and the cloud dispersal by the O stars is significant. We conclude that cloud–cloud collision offers a possible explanation of the mini-starburst over a 100 pc scale.

The stellar population of the blue compact dwarf galaxy SBS 1415+437 is investigated us- ing the archive database of the Hubble space telescope. The color index-magnitude diagram for stars reaches a magnitude of 29 m in the V and I bands. It comprises young main-sequence stars, blue and red supergiants, and the old population of red giant branch and asymptotic giant branch. The tip of the red giant branch (TRGB) was used to calculate the distance modulus, which turned out to be m-M = 30.65 0.08 m . The corresponding distance to the galaxy is D = 13.5 1.0 Mpc. The youngest stars are distributed irregularly near the bright H II region in the southwest part of SBS 1415+437. The old popu- lation occupies a larger area, it is distributed more evenly and forms the galactic halo. The spatial distri- bution of young stars shows that the star formation in the galaxy spread in the direction from northeast to southwest over the last 5 10 7 yr with an average rate of 60 km/s. The TRGB of SBS 1415+437 was found to be appreciably shifted to the blue range: (V - I)TRGB 1.30. The galaxy age turns out to be not smaller than the age of Galactic globular clusters (about 10 10 yr), provided that the galaxy originally had a very low metallicity (our photometric estimate is (Fe/H) = -2.4). If the metallicity of SBS 1415+437 changed almost not at all in the course of evolution and was equal to (Fe/H) = -1.3 (as estimated from the emission lines of ionized gas), the galaxy age is no more than 2 10 9 yr.

We studied star formation activities in the molecular clouds in the Large Magellanic Cloud. We have utilized the second catalog of 272 molecular clouds obtained by NANTEN to compare the cloud distribution with signatures of massive star formation including stellar clusters, and optical and radio HII regions. We find that the molecular clouds are classified into three types according to the activities of massive star formation; Type I shows no signature of massive star formation, Type II is associated with relatively small HII region(s) and Type III with both HII region(s) and young stellar cluster(s). The radio continuum sources were used to confirm that Type I GMCs do not host optically hidden HII regions. These signatures of massive star formation show a good spatial correlation with the molecular clouds in a sense they are located within ~100 pc of the molecular clouds. Among possible ideas to explain the GMC Types, we favor that the Types indicate an evolutionary sequence; i.e., the youngest phase is Type I, followed by Type II and the last phase is Type III, where the most active star formation takes place leading to cloud dispersal. The number of the three types of GMCs should be proportional to the time scale of each evolutionary stage if a steady state of massive star and cluster formation is a good approximation. By adopting the time scale of the youngest stellar clusters, 10 Myrs, we roughly estimate the timescales of Types I, II and III to be 6 Myrs, 13 Myrs and 7 Myrs, respectively, corresponding to a lifetime of 20-30 Myrs for the GMCs with a mass above the completeness limit, 5 x 10^4 Msun.

We present HST NICMOS photometry of the resolved stellar population in the dwarf irregular galaxy NGC 1569. The color-magnitude diagram (CMD) in the F110W and F160W photometric bands contains ~2400 stars with a formal photometric error 0.1 mag down to mF110W ≈ 23.5 and mF160W ≈ 22.5. The fiducial photometry has a completeness factor higher than 50% down to mF110W ≈ 21.5 and mF160W ≈ 20.0. We describe the data processing required to calibrate the instrumental peculiarities of NICMOS. Two different packages (DAOPHOT and StarFinder) for PSF-fitting photometry are used to strengthen the photometric results in the crowded stellar field of NGC 1569. The resulting CMD is discussed in terms of the major evolutionary properties of the resolved stellar populations. For a distance modulus of (m - M)0 = 26.71 and a reddening of E(B-V) = 0.56, our CMD samples stars down to ~0.8 M⊙, corresponding to look-back times of more than 15 Gyr (i.e., an entire Hubble time). This is a clear indication of star formation activity in NGC 1569 spanning an entire Hubble time. The metallicity of the reddest red giant branch (RGB) stars is in better agreement with Z = 0.004 as measured in H II regions than with Z = 0.0004, as expected from the stellar ages. However, the presence of—yet undetected—very metal-poor stars embedded in the stellar distribution around mF110W = 22.75 and mF110W - mF160W = 1.15 is not ruled out. The youngest stars (50 Myr) are preferentially found around the two central super star clusters, whereas the oldest population has a more uniform spatial distribution. A star formation rate per unit area of 1 M⊙ yr-1 kpc-2 and a mass formed in stars of ~ 1.4 × 106 M⊙ in the last 50 Myr are derived from the CMD. The near-infrared (NIR) CMD places strong constraints on the lower limit of the onset of star formation in NGC 1569. The exceptionally high crowding in the NICMOS images of NGC 1569 is a challenge for photometric analysis. As a result, optical and NIR images of NGC 1569 sample different populations and cannot be cross-correlated. Nevertheless, we demonstrate the consistency of the star formation histories derived from the optical and NIR CMDs.

We present the results of a large scale near-infrared (J, H, K) survey of the star forming region NGC 2264. The survey covers 48' (Dec) X 27' (RA) with a completness limit of 15.5, 14.5, and 14 magnitudes at J, H, and K respectively. Complementary observations of NGC 2023, NGC 2068 and NGC 7023 are also discussed. Young stars in NGC 2264 are concentrated in two clusters. The northern cluster is highlighted by the bright star S Mon. The infrared source NGC 2264 IRS, the molecular outflow NGC 2264D and the Cone Nebula are associated with the southern cluster. The northern cluster is divided into two subclusters, the southern cluster has three subclusters. The luminosity functions (LFs) of NGC 2264 subclusters have slopes in the range 0.28 - 0.30. This is also the case for NGC 2068. NGC 2023's LF is shallower (0.21). There is a break in the LF of NGC 7023 around K=10.5 magnitudes. All LFs are consistent with a Salpeter IMF and a Miller-Scalo IMF. For NGC 7023, we obtain a better agreement with a Miller-Scalo IMF. The differences in LF can be attributed to cluster ages. NGC 2023 is approxiamtely one million years old. NGC 7023 is old enough for its LF to be consistent with a cluster of main sequence stars. The NGC 2264 clusters and NGC 2068 have ages intermediate between NGC 2023 and NGC 7023. A large fraction of LFs have turn-offs at their faint end. These can be interpreted as turn-offs at the low mass end of the corresponding initial mass function. We present a new optical-infrared color-color diagram (V-J vs J-K) that is more efficient at detecting stars with infrared excesses than the infrared color-color diagram (V-H vs H-K). The luminosity function of infrared excess stars in NGC 2264 peaks around K=11 magnitudes. Finally, we report the detection of a pedestal stellar population in NGC 2264. Dissipation of the main clusters could explain the properties of this stellar population.

We present a Giant Molecular Cloud (GMC) catalog toward M33, containing 71 GMCs in total, based on wide field and high sensitivity CO(J=3-2) observations with a spatial resolution of 100 pc using the ASTE 10 m telescope. Employing archival optical data, we identify 75 young stellar groups (YSGs) from the excess of the surface stellar density, and estimate their ages by comparing with stellar evolution models. A spatial comparison among the GMCs, YSGs, and HII regions enable us to classify GMCs into four categories: Type A showing no sign of massive star formation (SF), Type B being associated only with HII regions, Type C with both HII regions and <10 Myr-old YSGs and Type-D with both HII regions and 10--30 Myr YSGs. Out of 65 GMCs (discarding those at the edges of the observed fields), 1 (1%), 13 (20%), 29 (45%), and 22 (34%) are Types A, B, C, and D, respectively. We interpret these categories as stages in a GMC evolutionary sequence. Assuming that the timescale for each evolutionary stage is proportional to the number of GMCs, the lifetime of a GMC with a mass >10^5 Mo is estimated to be 20--40 Myr. In addition, we find that the dense gas fraction as traced by the CO(J=3-2)/CO(J=1-0) ratio is enhanced around SF regions. This confirms a scenario where dense gas is preferentially formed around previously generated stars, and will be the fuel for the next stellar generation. In this way, massive SF gradually propagates in a GMC until gas is exhausted.

High mass star formation and the evolution of HII regions have a substantial impact on the morphology and star formation history of molecular clouds. The HII region Gum 48d, located in the Centaurus Arm at a distance of 3.5 kpc, is an old, well evolved HII region whose ionizing stars have moved off the main sequence. As such, it represents a phase in the evolution of HII regions that is less well studied than the earlier, more energetic, main sequence phase. In this paper we use multi-wavelength archive data from a variety of sources to perform a detailed study of this interesting region. Morphologically, Gum 48d displays a ring-like faint HII region associated with diffuse emission from the associated PDR, and is formed from part of a large, massive molecular cloud complex. There is extensive ongoing star formation in the region, at scales ranging from low to high mass, which is consistent with triggered star formation scenarios. We investigate the dynamical history and evolution of this region, and conclude that the original HII region was once larger and more energetic than the faint region currently seen. The proposed history of this molecular cloud complex is one of multiple, linked generations of star formation, over a period of 10 Myr. Gum 48d differs significantly in morphology and star formation that the other HII regions in the molecular cloud; these differences are likely the result of the advanced age of the region, and its different evolutionary status.

J , H and K' images obtained with the Canada-France-Hawaii Telescope were used to investigate the stellar contents of the asymptotic giant branch (AGB) in the dwarf elliptical galaxy NGC 185. The bright parts of and color-magnitude diagrams consist of a group of bright blue stars, a dominant population of M-giants and a red C star population. There were 73 C stars with a mean magnitude of , corresponding to , and mean colors of and . The number ratio of C stars to M-giants was estimated to be without any radial gradient from the center of NGC 185. The and color distributions of AGB stars showed an M-giant peak and blue and red tails, where the latter two correspond to AGB stars younger than those along M-giant peak and C stars. The bolometric luminosity functions of M-giants and C stars indicate that the M-giant AGB sequence has terminated at , while the most luminous C star has . The bolometric luminosity function of C stars in NGC 185 is very similar to that of recent literature values derived from Vi band photometry. The logarithmic slope of the luminosity function for bright M-giant stars was estimated to be in K band. Theoretical isochrone models, compared with the observed near-infrared photometric properties of AGB stars, indicate that star formation in NGC 185 has a wide range of ages with possibly two different epochs of star formation.

The Canada-France-Hawaii Telescope Adaptive Optics Bonnette has been used to obtain high angular resolution JHK images of the centers of the metal-poor globular clusters NGC 5272 (M3), NGC 6205 (M13), NGC 6287, and NGC 6341 (M92). The color-magnitude diagrams (CMDs) derived from these data include the upper main sequence and most of the red giant branch and agree with published photometric measurements of bright stars in these clusters. The photometric accuracy is limited by point-spread function variations, which introduce systematic errors of a few hundredths of a magnitude near the reference star. The NGC 6287 CMDs are of particular interest, as they are free of the scatter caused by foreground stars and differential reddening that have plagued previous efforts to study the stellar content of this inner spheroid cluster. The horizontal branch (HB) of NGC 6287 contains stars spanning a broader range of temperatures than that of NGC 6341. NGC 6287 is thus the only known member of the metal-poor inner spheroid to deviate from the HB morphology versus metallicity relation defined by other clusters in the inner Galaxy. The clusters are paired according to metallicity, and the near-infrared CMDs and luminosity functions are used to investigate the relative ages within each pair. The near-infrared CMDs provide the tightest constraints on the relative ages of the classical second-parameter pair NGC 5272 and NGC 6205 and indicate that these clusters have ages that differ by no more than ±1 Gyr. These results thus support the notion that age is not the second parameter. We tentatively conclude that NGC 6287 and NGC 6341 have ages that differ by no more than ±2 Gyr. However, the near-infrared spectral energy distributions of stars in NGC 6287 appear to differ from those of stars in outer halo clusters, bringing into question the validity of this age estimate.